This invention relates to surface acoustic wave withdrawal weighted interdigital transducers and in particular to an electrode thinning technique that achieves mass loading reductions in such devices.
Surface acoustic wave (SAW) technology has become increasingly attractive for various signal processing applications, primarily because SAW devices can be fabricated to be compact, lightweight, and reliable using well established planar techniques. Many analog filter functions, not easily synthesized with classical techniques, can be realized with SAW transversal filters.
Over the years the length-weighted or apodized interdigital transducer has been the most important part of a SAW filter. A typical filter arrangement comprises a weighted interdigital transducer and an unweighted interdigital transducer. The weighted interdigital transducer accepts an electrical signal and converts it into acoustic waves. The second transducer samples, weights and integrates the incoming acoustic energy. The overall process is linear and reciprocal and can be expressed in terms of mathematical convolution or equivalently EQU V.sub.o /V.sub.i = H (j.omega.) = H.sub.i (j.omega.)H.sub.2 (j.omega.)e.sup.-j.omega..tau. s (1)
Where .tau..sub.s is the acoustic propagation delay between transducers. Because the unapodized transducer is often wideband, H(j.omega.) is approximately equal to the apodized response H.sub.1 (j.omega.) which is the Fourier transform of the apodization function of the transducer.
Even though apodization is conceptually simple, it is very difficult to achieve sidelobe levels below -50dB because of fabrication errors and diffraction distortions. Also, cascaded apodized IDT's fails to yield an overall response that equals the product of the individual responses (Eq. (1)). These drawbacks motivated the search for alternative weighting techniques such as phase weighting, capacitive weighting, series weighting and withdrawal weighting. Withdrawal weighting shows the greatest promise for complementing or replacing length apodized filters.
The basic objective of withdrawal weighting is to synthesize a desired bandpass response by selectively removing electrodes from what otherwise would be a uniform periodic array of electrodes. Unlike capacitive weighting and phase weighting, withdrawal weighting does not complicate or impose harsh requirements on device fabrication. Since a withdrawal weighted IDT launches a nearly plane acoustic wavefront, two of these IDT's can be placed in series within the same filter with minimal spectral and diffraction distortion.
Although withdrawal weighted transducers represent an improvement over devices using other weighting schemes they also have some undesirable characteristics. Most notable is the passband distortion which arises due to mass loading. Conventional withdrawal weighted transducers exhibit a velocity discontinuity between the metal electrodes and the substrate. There is also an impedance discontinuity which causes surface waves to be partially reflected by electrodes within the array. These internal reflections result in the aforementioned passband distortion. The present invention is directed toward providing withdrawal weighted interdigital transducers in which these undesirable characteristics are minimized.